jdbc连接数据库候，对数据进行访问，访问正常当出现如下警告：

WARN: Establishing SSL connection without server's identity verification is not recommended. According to MySQL 5.5.45+, 5.6.26+ and 5.7.6+ requirements SSL connection must be established by default if explicit option isn't set. For compliance with existing applications not using SSL the verifyServerCertificate property is set to 'false'. You need either to explicitly disable SSL by setting useSSL=false, or set useSSL=true and provide truststore for server certificate verification.

根据警告字面意思，将数据库连接地址进行修改：

jdbc:mysql://localhost:3306/db_book   改为   jdbc:mysql://localhost:3306/db_book?useSSL=false